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Neutrons Reveal that Spin Glass can Mimic Quantum Spin Liquid

17 May 2018

Quantum spin liquids represent a novel state of matter in which the spins do not arrange themselves in an ordered pattern as they do in a conventional magnet. This state features spins with long-range entanglements and fractional excitations, and thus can find applications in quantum computation and communication. In 1973, the Nobel laureate Philip W. Anderson proposed a model on a triangular lattice for such a state, but up to now, this has not been realized experimentally.

An international team of scientists led by Prof. Jinsheng Wen from Nanjing University in China has now found that in labelling a material as a quantum spin liquid, one must be aware that other states may also produce some spin-liquid-like features, for example, spin glass.

In the past two years, evidence has accumulated indicating that compound YbMgGaO4 with a triangular-lattice structure is in fact the quantum spin liquid proposed by Anderson. To test whether this argument is correct, the researchers measured YbMgGaO4 and its sister compound YbZnGaO4 with neutrons combined with various other techniques. Results are similar for both compounds, and some resemble those of a quantum spin liquid.

For example, from inelastic neutron scattering, mainly performed on the cold three-axes spectrometer PANDA, operated by the Jülich Centre for Neutron Science at its outstation at the Heinz Maier-Leibnitz Zentrum, they observed the quasi-continuous excitation spectrum, which is considered to be the conclusive evidence for a spin-liquid state. In fact, by combining results from susceptibility, specific heat, and thermal conductivity measured at temperatures of around 0.05 Kelvin, close to absolute zero, as well as theoretical calculations, they found that the true ground state is indeed a spin-glass. The team showed that a spin glass mimics a quantum spin liquid in many ways, but is induced by the disordered arrangements of the atoms, instead of the quantum fluctuations needed for the latter.